The wood and grain quality is absolutely crucial for an easy build. If you have lower quality wood, you can probably get away with it, but it takes much longer and will probably be heavier and not as durable. We use mostly thin sticks in a SOF kayak so a little knot can weaken the material by a lot. That needs to be taken care of. You can scarf it or laminate to make it usable anyway. If you have no choice, use your imagination and work from there!
So, having that out of the way, here are some examples.
Cockpit coaming and ribs
Here you can use any hardwood which can be heated and bent into a rounded shape. Green, freshly cut wood is much better than old, dried wood. We use Ash and White Oak. It seems easier to get straight Ash than Oak in Sweden and the Ash kan even be air dried, as long as it’s straight grained.
For both Ash and Oak, it helps if you soak it for a few days before bending. Some people like to add machine wash fabric softener for the water to penetrate better. Some even say kiln dried Ash can be used if you soak it first.
Most builders make the Masik out of hardwood, but you can also laminate to make it strong. I have made Spruce Masiks which are durable enough to jump on. You just have to make sure there are no knots in bad places. So how do you laminate it? You can make thin, flat strips or just glue a few planks together and band-saw the form. Anything goes but you have to make an informed decision about the durability.
Gunwhales, stringers, keel, stems and deck beams
They are all made of a lighter, softer wood. You can laminate hard wood for extra durability wherever you want, but it adds weight, time and effort. I have used Spruce, Fir, Cedar and now Paulownia which is a very light wood and doesn’t give people allergic reactions like Cedar can. Just make sure it’s knot free, although with Spruce you can get away with the tiniest knots (pärlkvist).
Rot in wood is generally not a problem in Scandinavia if you treat it with oil. However, in warmer climates and frequent use you might want wood which is rot resistant, like Cedar.
Nylon and/or polyester? Uncoated Ballistic Nylon is harder to get than one might think. It is probably too cheap to sell as-is so all manufacturers coat it with PU and sell it as a water proof backpack fabric or something. You want something which gives the PU enough surface to saturate. It should also have a tight weave to not have strands being pulled apart when you stitch you kayak skin. Somewhere between 250-300 g/m2 and around 840D is what we use.
String and thread
For lashing of the stringers, keel and gunwhales you can use any string which is flat, ie. not spun or twisted. There is a waxed nylon string called Artificial Sinew which works the best. It is probably not hard to find but will cost you a premium. An example: you find Artificial Sinew on Amazon for $20. The store charges $15 to ship it. Swedish customs charge you 75SEK for handling, an import tax from some countries and VAT of 25%. It quickly adds up, since you may have ordered clamps, a Japanese saw, a slick plane, block plane etc. from Amazon too…
I found Artificial Sinew in India and got a package of six rolls which will last me 15 more kayaks.
For thread you need a good, strong upholstery thread which is made of polyester or nylon.
There are a number of different compounds. See post here.
Linseed oil takes ages to cure. If you have time, go ahead, but using a good, modern wood oil with solvents will get your kayak ready faster. Just make sure it penetrates well, especially where the grain ends, and that the frame is properly dried off with a cloth immediately after oiling. If you have residue oil where the skin fabric touches the frame, you may have adhesion problems with the Polyurethane. It will not be worth it, so be careful.